1. Field of the Invention
This invention relates to a process for producing purified diammonium phosphate from wet process phosphoric acid. More particularly it relates to a process for producing industrially usable ammonium phosphate directly from wet process phosphoric acid with a high yield.
2. Description of the Prior Art
Diammonium phosphate obtained by reacting ammonia with wet process phosphoric acid contains a large amount of impurities originated from the above phosphoric acid; hence it can be used as fertilizers, but cannot be used for industrial uses, as it is. For producing industrial purified diammonium phosphate from wet process phosphoric acid, it is necessary to purify the phosphoric acid in advance according to a process of extracting phosphoric acid from wet process phosphoric acid according to solvent process, followed by reacting ammonia with the resulting extract to obtain diammonium phosphate containing a small amount of impurties.
However, this extraction process cannot be regarded as an industrially advantageous process due to its complicated steps. On the other hand, as to various compounds stepwise formed by the reaction of wet process phosphoric acid with ammonia, "Studies on Chemical fertilizers" (written by Ando and Akiyama, issued as a reprint on May 20, 1976, from Nissin Syuppan K. K.) has suggested for the present a possibility of producing industrial ammonium phosphate from a mother liquor of ammonium phosphate obtained by neutralizing wet process phosphoric acid with ammonia under certain neutralization conditions to form (FeAl)NH.sub.4 H.sub.2 (PO.sub.4).sub.2.1/2H.sub.2 O (abbreviated to Q compound) or (FeAl)NH.sub.4 HF.sub.2 PO.sub.4 (abbreviated to S compound) to thereby obtain the mother liquor of ammonium phosphate having Fe, Al and F contained in wet process phosphoric acid removed therefrom. However, this process also has a drawback that it requires such prolonged steps as stepwise pH control, filtration step and crystallization step, which results in increase of energy consumption.
On the other hand, according to French patent No. 1,248,055 (Prayon process), an industrial purified product is obtained by reacting ammonia with wet process phosphoric acid, partly forming crystals of diammonium phosphate in the reaction liquid, separating the crystals, cutting a part of the resulting mother liquor to reduce the amount of impurities originated from wet process phosphoric acid, thereafter concentrating the mother liquor to crystallize raw crystals, and recrystallizing the raw crystals. The yield of purified diammonium phosphate obtained according to the process is about 40.about.50% based on P.sub.2 O.sub.5 content in wet process phosphoric acid as raw material, and the remainder of P.sub.2 O.sub.5 ultimately is used as ammonium phosphate for fertilizers.
The yield of ammonium phosphate based on phosphoric acid in the above description is defined as "percentage recovery" as follows: ##EQU1##
As described above, when wet process phosphoric acid is directly neutralized, the yield of purified ammonium phosphate based on phosphoric acid is considerably low.
In view of the above problems of the prior art, the present inventors have made extensive research in order to find a process for producing diammonium phosphate with a high yield i.e. a high percentage recovery by direct neutralization of wet process phosphoric acid.
As a result, we have found that when wet process phosphoric acid and ammonia are reacted at 50.degree..about.80.degree. C. while the molar ratio of NH.sub.3 / H.sub.3 PO.sub.4 is kept at 1.65-1.90, followed by separating the resulting slurry containing diammonium phosphate crystals and an insoluble sludge into a slurry containing the former and a slurry containing the latter, treating the former slurry according to a conventional process to recover diammonium phosphate, while treating the latter slurry according to a process described later to remove the sludge, combining the resulting filtrate with that obtained at the time of recovery of diammonium phosphate, and concentrating the mixture, to recover diammonium phosphate, and combining the two portions of diammonium phosphate whereby diammonium phosphate can be recovered with a high yield. The above-mentioned process for treating the slurry containing an insoluble sludge refers to a process of heating the slurry to 60.degree..about.95.degree. C. to dissolve diammonium phosphate contained therein, followed by centrifugal separation and recovery of the resulting filtrate.
As is apparent from the above description, an object of the present invention is to provide a process for obtaining diammonium phosphate from wet process phosphoric acid with a high yield. Another object of the present invention is to provide diammonium phosphate produced according to the above process.